1. Field of the Invention
The present invention relates to an image line sensor unit, photosensors for use in the line sensors and methods of making the photosensors.
2. Description of the Prior Art
In conventional photoelectric conversion apparatus used as optical input units of image data processing apparatus such as facsimile devices, reproduction devices, or character readers, it is well known to use photosensors as photoelectric conversion elements. Recently, high sensitivity image readers are composed of line sensors each including a one-dimensional array of photosensors. One example of such a line sensor is a planar photoelectric photosensor having a pair of opposing electrodes made of metal or the like disposed so as to form a spacing which forms a photoreceptor on a photoelectric layer which contains amorphous silicon (referred to as a-Si hereinafter, and, among others, amorphous silicon which especially contains atoms such as hydrogen or halogens, e.g., fluorine, is referred to as a-Si(H,X) hereinafter).
Methods of preparing a-Si(H,X) which is used for making such a photosensor are known, examples being plasma CVD, reactive sputtering, or ion plating. All of these methods use glow discharge to expedite reaction. In any of these methods, however, it is necessary to form an a-Si(H,X) film with relatively low discharge power in order that the film has high quality and high photoconductivity. Photoconductive layers resulting from formation of films with such low discharge power, however, have insufficient adhesion to their substrates made of glass or ceramics and the films are likely to peel off when the photoconductive layers are subjected to a subsequent photolithography process for forming the electrodes.
In order to prevent this peeling-off of films, a method of roughening a surface of the substrate and depositing a-Si(H,X) on the resulting surface has been employed. That is, the substrate surface is previously etched chemically for example with hydrofluoric acid, or scratched physically, for example, with a brush.
However, this method has the following drawbacks:
(1) The system associated with the washing line has a complicated structure and is expensive when a chemical such as hydrofluoric acid is used;
(2) It is difficult to control the degree of the ruggedness of the substrate surface; and
(3) Microscopic defects are likely to occur at the substrate surface when same is roughened. The characteristics of an a-Si(H,X) film deposited on the microscopic defects and hence the characteristics of the resulting photoconductive layer are likely to vary from place to place.
Therefore, when an image line sensor unit is formed by photosensor units such as mentioned above, a correction circuit which eliminates a large discrepancy between respective bit signals must be additionally provided, thereby increasing cost.